MY GRANDFATHER WAS not one for self-gratification. In the Islamic Republic of Iran he couldn’t drink even if he’d wanted to. My father told me that he ‘probably had six whiskies in his entire life, one for each time he visited us in England’. He kept fit, waking every morning at 5 a.m. to go hiking in the foothills of the Alborz Mountains in northern Tehran. He didn’t smoke. And he had a good diet, eating plenty of fish, pomegranates, pistachio nuts and the vegetable-infused, rosewater-scented stews typical of Persian cuisine.

He lived a predominantly stress-free life too. A property developer in the affluent province of Shemiran, Abbas had inherited a fortune from his own father, Shaban, a logger and well-known figure in Tehran. Abbas was therefore free of financial worries, and in truth didn’t need to work.

All told, my grandfather’s decidedly salubrious life sheds little light on the cause of his illness. And this, I was beginning to realise, was a recurring theme: Arnold, the cultivated and bookish South African I met in London, lived a healthy life, as had Carol, the upbeat and proactive early-onset patient from Coventry. So what–if anything–destined a person towards dementia? More and more it was looking like the Californian neuroscientist Arthur Toga was right when he replied simply ‘bad luck’, in response to the same question posed by Terry Pratchett in 2008. Sardonically, Toga went on to say that ‘this is an equal opportunity disease’.

But as I read the studies on lifestyle countermeasures–and there are many–it became clear to me that there is still cause for hope. Though much of the evidence is preliminary and inconclusive, and sometimes merely anecdotal, considerations for stress, diet, exercise, cognitive training and even sleep are gaining scientific ground. And so I stepped into the uncertain world of preventive healthcare with cautious optimism.